I. Field of the Invention
This invention relates to a gas separation membrane having good gas permeability, selectivity, durability and anti-scratching properties.
II. Background of the Invention
Recently, gas separation by means of a membrane, especially production of oxygen-enriched air by gas separation, has attracted attention. The membrane practically used for the gas separation must have excellent gas-separating properties and a high gas permeability. To satisfy these requirements, various separation membranes have recently been invented by studying the membrane materials, membrane structures, and processes of producing the membranes.
As for the membrane materials, the following are exemplary:
(1) In Japanese Patent Disclosure (Kokai) Nos. 146277/79, 40413/81, 92925/81 and 92926/81, a gas separation membrane consisting essentially of a polyolefin such as poly(4-methyl-1-pentene) or the like, and the production process thereof are disclosed. Further, in Japanese Patent Publication (Kokoku) No. 30169/84, a gas separation membrane formed from a composition consisting essentially of polymethylpentene and polyorganosiloxane/polycarbonate copolymer is disclosed. In Japanese Patent Disclosure (Kokai) No. 28605/81, a gas permeation membrane comprising as the major component an olefin-based macromolecule partially crosslinked with polyorganosiloxane is disclosed.
(2) In Japanese Patent Disclosure (Kokai) No. 32903/84, a separation membrane formed from a modified poly(4-methyl-1-pentene) prepared by graft-copolymerizing an organosilane derivative of an unsaturated carboxylic acid is disclosed.
(3) Japanese Patent Publication (Kokoku) No. 25022/68 teaches a gas separation membrane made of a polyarylene oxide, and Japanese Patent Disclosure (Kokai) No. 216802/85 teaches a gas separation membrane made of a silylated polyphenylene oxide.
(4) As examples in which crosslinked polyarylene oxide is used as a gas separation membrane, Japanese Patent Disclosure (Kokai) Nos. 222203/84 and 53373/87 disclose a gas separation membrane formed from a polyarylene oxide crosslinked by aminoalkyl-substituted polysiloxane, and Japanese Patent Disclosure (Kokai) No. 51525/85 discloses a crosslinked polyarylene oxide membrane prepared by a reaction of a halogenated polyarylene oxide with a crosslinking agent such as ammonia.
(5) The gas permeability of poly(2-trimethoxysilyl-1,3-butadiene) was discussed in Polymer Preprints, Japan 35 (3), 425 (1986).
On the other hand, as for the membrane structures, the following are exemplary:
(6) Japanese Patent Disclosure (Kokai) No. 112802/84 discloses a semipermeable membrane comprising a layer of silicone-based macromolecule and a layer of macromolecule such as polyolefin and polydiene of which the glass transition temperature is not higher than room temperature, and the two layers are supported on a porous support.
(7) Japanese Patent Disclosure (Kokai) No. 59214/84 discloses a semipermeable membrane comprising a first membrane formed on a porous support, which is formed from a material having an oxygen permeability coefficient of 10.sup.-8 -10.sup.-7 (cm.sup.3.cm/cm.sup.2.sec.cmHg), such as polyorganosiloxane and a second membrane superposed on the first membrane, which is made of a material having a high separation ability of oxygen from nitrogen, such as poly(vinyl pivalate).
However, the above membranes (1) have a low selectivity in separating a particular gas from a gas mixture.
The above membrane material (2) has an improved selectivity, but it has problems in that the strength of the membrane is small and the feasibility of forming a thin film is not good.
As to the above gas separation membranes (3), the polyarylene oxide does not have a crosslinked structure, and so the film-formability and the durability of the membrane are low.
As to the above membranes (4), it is necessary to add a crosslinking agent in a post-treatment step for crosslinking the halogenated polyarylene oxide, so that the production process is complicated.
The above membrane (5) has a low strength and, in particular, reproducible measurement results have not yet been obtained in the glassy state of the membrane material. Further, the measurement results in a rubbery state so that a separation factor .alpha. (=PO.sub.2 /PN.sub.2) of the membrane material is as low as 3.
As to the above semipermeable membranes (6) and (7), since the top layer made of a polymer with high gas separation ability has a low mechanical strength, defects such as pinholes are likely to be produced in the top layer due to the friction when the membrane is wound, or due to the contact with a spacer which is necessary to assure a flow path of the gas when the membrane is incorporated in a module, so that the gas-separation ability thereof is reduced.